A wireless communication system including a base station and a mobile station has been known (see, for example, Patent Literatures 1-4 and Non-Patent Literatures 1 and 2). The mobile station includes a receiving apparatus that receives a wireless signal from the base station. The base station includes a receiving apparatus that receives a wireless signal from the mobile station. The mobile station wirelessly communicates with the base station in a wireless area that the base station forms.
For example, LTE Release 10 prescribed by 3GPP proposes a scheme, called eICIC, of controlling interference between cells. Here, the term 3GPP is an abbreviation for Third Generation Partnership Project; the term LTE is an abbreviation for Long Term Evolution; and the term eICIC is an abbreviation for Enhanced Inter-Cell Interference Coordination.
For example, the eICIC is assumed to be carried out in cases where, as illustrated in FIG. 1, the pico base station 92 forms a picocell C2 inside the macrocell C1 formed by a macro base station 91. In this case, a wireless signal that a mobile station 93 positioned in the picocell C2 receives from the pico base station 92 is largely interfered by a wireless signal transmitted in the macrocell C1. Hereinafter, a wireless signal that a mobile station 93 positioned in the picocell C2 receives from the pico base station 92 is referred to as a desired signal while a wireless signal transmitted in the picocell C1 is referred to as an interfering wave.
Considering the above, as illustrated in FIG. 2, eICIC sets a time period P1 to transmit an Almost Blank Subframe (ABS) in the macrocell C1 and transmits data to the mobile station 93 in the picocell during the time period P1.
An ABS includes a reference signal (e.g., a CRS), a synchronization signal (e.g., a PSS and an SSS), and broadcast information. The term CRS is an abbreviation for Cell-Specific Reference Signal; the term PSS is an abbreviation for Primary Synchronization Signal; and the term SSS is an abbreviation for Secondary Synchronization Signal. For example, broadcast information is transmitted through a Physical Broadcast Channel (PBCH). An ABS is a subframe in which data is not allocated to a wireless resource different from a wireless resource allocated thereto a reference signal, a synchronization signal, and broadcast information.
This can suppress influence of an interfering wave on the reception quality of the mobile terminal 93 during the time period P1, so that the reception quality of the mobile terminal 93 can be enhanced. Consequently, a region in which the picocell C2 accommodates the mobile terminal 93 can be expanded. This expansion is also called Cell Range Expansion (CRE).
Here, a reference signal, a synchronization signal, and broadcast information included in an ABS degrade the reception quality of the mobile station 93. To solve this disadvantage, the LTE Release 11 prescribed by 3GPP proposes the scheme of Further Enhanced ICIC (feICIC).
The scheme of feICIC notifies a mobile station of information of an interfering cell, which transmits a wireless signal that causes an interfering wave. Information related to an interfering cell is also referred to as CRS Assistance Information. Besides, the feICIC cancels a reference signal, a synchronization signal, and broadcast information included in an ABS, which is transmitted in a macrocell, in a reception process performed by a mobile station. Thereby, the reception quality of the mobile station can be improved, so that the region in which the picocell can accommodate a mobile terminal can be expanded.
A receiving apparatus that cancels a reference signal, a synchronization signal, and broadcast information from the ABS transmitted in the macrocell is also called an Interference Canceling (IC) receiving apparatus. Cancelling a reference signal, a synchronization signal, and broadcast information are called CRS-IC, PSS/SSS-IC, and PBCH-IC, respectively.
Here, the overview of CRS-IC will now be described as an example. A received signal for the l-th symbol time of the k-th subcarrier in OFDM is expressed by Expression 1. The term OFDM is an abbreviation for Orthogonal Frequency-Division Multiplexing. The l-th symbol time is a time corresponding to the l-th OFDM symbol along the time axis.
                              y          ⁡                      (                          k              ,              l                        )                          =                                            H              ⁡                              (                                  k                  ,                  l                                )                                      ⁢                          V              ⁡                              (                                  k                  ,                  l                                )                                      ⁢                          x              ⁡                              (                                  k                  ,                  l                                )                                              +                                    ∑                              i                =                1                                            N                cell                                      ⁢                                                  ⁢                                                            H                  i                                ⁡                                  (                                      k                    ,                    l                                    )                                            ⁢                                                x                  i                                      (                    CRS                    )                                                  ⁡                                  (                                      k                    ,                    l                                    )                                                              +                      n            ⁡                          (                              k                ,                l                            )                                                          [                  Expression          ⁢                                          ⁢          1                ]            
Here, the term y(k, l) represents a received signal and is a Nr×1 vector. The term Nr represents the number of receiving antennas that the mobile station uses. The term H(k, l) represents a channel between the base station and the mobile station in a serving cell, and is an Nr×Nt matrix. A serving cell is a wireless area accommodating the mobile station. The term Nt represents the number of transmitting antennas that the base station uses in the serving cell.
The term V(k, l) represents an Nt×Nstream transmitting precoding matrix in the serving cell. The term Nstream represents the number of streams contained in a desired signal. A desired signal is a wireless signal that the base station transmits to the mobile station in the serving cell and is also referred to as a transmitted signal. The term x(k, l) represents the desired signal and is an Nstream×1 vector.
The term Ncell represents the number of interfering cells, which are wireless areas different from the serving cell. The term Hi(k, l) represents a channel between the base station and a mobile station in an i-th interfering cell and is an Nr×Nt,i matrix. The term Nt,i represents the number of transmitting antennas that the base station uses in the i-th interfering cell.
The term xi(CRS)(k, l) represents a CRS transmitted in the i-th interfering cell and is an Nt,i×1 vector. The term n(k, l) represents Additive White Gaussian Noise (AWGN) and is an Nr×1 vector.
A mobile station estimates the channel He,i between the base station and a mobile station in the i-th interfering cell. A channel between the base station and a mobile station in an interfering cell is called an interfering channel. As denoted in Expression 2, the mobile station cancels a component derived from a reference signal transmitted in an interfering cell from the received signal y by subtracting a signal obtained by multiplying the reference signal xi(CRS) by the estimated interfering channel He,i from the received signal y. The signal obtained by multiplying the reference signal xi(CRS) by the estimated interfering channel He,i is also referred to as the replica signal of a reference signal.
                                          y            PC                    ⁡                      (                          k              ,              l                        )                          =                              y            ⁡                          (                              k                ,                l                            )                                -                                    ∑                              i                =                1                                            N                cell                                      ⁢                                                  ⁢                                                            H                                      e                    ,                    i                                                  ⁡                                  (                                      k                    ,                    l                                    )                                            ⁢                                                x                  i                                      (                    CRS                    )                                                  ⁡                                  (                                      k                    ,                    l                                    )                                                                                        [                  Expression          ⁢                                          ⁢          2                ]            
The term yPC represents a signal obtained by canceling a component derived from a reference signal transmitted in an interfering cell from a received signal.
The mobile station carries out a reception process on the basis of the received signal yPC after undergoing the canceling. Examples of the reception process are estimation of a channel in the serving cell, demodulation of the received signal, and error correction decoding on the received signal.    [Patent Literature 1] Japanese Laid-open Patent Publication No. 2013-98941    [Patent Literature 2] Japanese National Publication of International Patent Application No. 2012-529786    [Patent Literature 3] Japanese National Publication of International Patent Application No. 2013-524736    [Patent Literature 4] Japanese National Publication of International Patent Application No. 2011-530247    [Non-Patent Literature 1] Baojin Li and two other persons, “Interference Cancellation for HetNet Deployment in 3GPP LTE-Advanced Rel-11”, Proceeding IEEE Vehicular Technology Conference, IEEE, p. 1-5, June 2013    [Non-Patent Literature 2] J. C. Henao and 15 other persons, “Advanced receiver signal processing techniques: evaluation and characterization”, Advanced Radio Interface Technologies for 4G Systems, [searched on Mar. 30, 2014], Internet, <URL:https://ict-artist4g.eu/projet/work-packages/wp2/deliverables/d2.2/final/d2.2-1/at download/file>